The present invention relates to a multiplexer/demultiplexer for use in mobile communication devices such as portable telephones.
FIG. 14 represents a circuit construction of a conventional multiplexer/demultiplexer. In FIG. 14, the circuit is comprised of a low-pass filter 101, a high-pass filter 102, phase shifters 103, 104, input terminals 105, 106, and an output terminal 107. Here, low-pass filter 101 and high-pass filter 102 are configured as xcfx80-shaped 3-stage circuits.
In a filter of this configuration, as the filter functions closer to short-circuiting as the signal frequency goes away from the pass band, it will not function as a multiplexer/demultiplexer by simply connecting each filter as it is. Accordingly, by connecting phase shifters 103, 104 as shown in FIG. 14 on the side the filters are mutually connected, the phase of a signal outside of the pass band is shifted by about 180 degrees thereby to open the circuit. Phase shifters 103 and 104 are comprised of a transmission line such as a strip line.
However, this configuration suffered serious problems of difficulty in miniaturization because of the larger size of transmission lines composing phase shifters 103, 104 compared with other components and of a large loss due to longer line lengths.
The present invention is directed to providing an easy-to-miniaturize, low loss, high-performance multiplexer/demultiplexer.
In order to achieve the above purpose, the multiplexer/demultiplexer of the present invention includes first to third ports each externally terminated with a pure resistance, in which the third port is made to be a common port by connecting a first resonant circuit between the first port and the ground. The first resonant circuit makes serial resonance at a frequency within a first frequency band or in its proximity. An inductive element is connected in series between the first port and the third port, and a second resonant circuit is connected between the second port and the ground. The second resonant circuit makes serial resonance at a frequency within a second frequency band which is different from the first frequency band or in its proximity. A capacitive element is connected in series between the third port and the second port. This way, it is made possible to easily obtain a small and high-performance multiplexer/demultiplexer with a simple circuit with an exceptionally small number of elements without using a transmission line which tends to cause a loss.